"The baby cried. Venus Express has begun its operational mission," said Jean-Pierre Cau, an official from EADS Astrium, the company that built the spacecraft's propulsion system, after receiving the signal.

Venus Express separated successfully from the Soyuz Fregat carrier rocket more than 90 minutes after its 0333 GMT launch at the start of a 163-day journey to Venus.

The European Space Agency's (ESA) first probe to the planet, Venus Express will explore its unusual stormy atmosphere and runaway global warming in the hope of better understanding Earth's greenhouse-gas problem.

It is hoped that the 1.27-tonne unmanned Venus Express orbiter will be able to use seven powerful instruments on board to map the planet's surface and weather system, looking at temperature variation, cloud formations, wind speeds and gas composition.

After a 163-day journey, the craft is scheduled to arrive off Venus in April, when it will be placed in an elliptical orbit, swooping to as low as 250 kilometres above the surface to a height of 66,000 kilometres.

The orbiter, whose total mission costs are 220 million euros, has enough fuel to operate for 1,000 Earth days.

Venus Express was to have been launched by a Russian Soyuz-Fregat rocket, operated by the company Starsem, from Baikonur on October 26.

But five days ahead of the scheduled launch, the ESA said "contamination" had been detected inside the fairing - the bullet-shaped hood that covers the payload on the top of the rocket - in final checks at Baikonur, AFP reports.